Method of magnetic field controlled shimming

Abstract

The present invention is directed to a method of shimming a magnet assembly of an MR imaging system such that a desired B0 field strength may be created with minimal inhomogeneities therethrough. With this method, sufficient shimming of the magnet assembly may be achieved without requiring mechanical variations to the magnet assembly after the magnet assembly has been assembled. The invention analyzes variations from the desired B0 field and inhomogeneities at a number of target points along the magnet assembly or B0 field. A comparison is then made at each point to determine a shimming or weighting factor such that the desired overall B0 field strength and targeted field homogeneity is achieved. Active and/or passive shim elements may then be incorporated into the magnet assembly at each target point to achieve the desired overall field strength and minimum overall field homogeneity.

What is claimed is:  
     
       1. A method of shimming a magnet for an MR imaging system, the method comprising the steps of: 
       creating a B 0  field about an imaging volume using a magnet assembly designed to generate a B 0  field at a pre-defined field strength;  
       determining a minimum acceptable field inhomogeneity for the B 0  field;  
       determining at least one of a field strength variation from the pre-defined B 0  field strength and an inhomogeneity variation from the minimum acceptable field inhomogeneity at a number of target point along the B 0  field; and  
       shimming at at least a portion of the number of target points such that an actual field inhomogeneity does not exceed the minimum acceptable field inhomogeneity and the actual B 0  field strength at least substantially approximates the pre-defined field strength after shimming.  
     
     
       2. The method of  claim 1  wherein the step of shimming comprises the step of adding one of a passive shim element and/or an active shim element to a magnet assembly corresponding to at least a portion of the number of target points. 
     
     
       3. The method of  claim 2  wherein the step of adding includes the steps of incorporating a shim coil into the magnet assembly at at least one of the number of target points and determining an amount of current to be applied to the shim coil to control contribution of magnetic field by the shim coil to the B 0  field. 
     
     
       4. The method of  claim 3  further comprising the step of constructing the shim coil such that harmonics of the magnetic field contributed by the shim coil offset inhomogeneities in the B 0  field. 
     
     
       5. The method of  claim 2  wherein the step of adding includes the step of incorporating one of a permanent magnet and an iron core into the magnet assembly at at least one of the number of target points, wherein the one of the permanent magnet and the iron core has a mass determined to be minimally sufficient to contribute to the B 0  field strength and offset inhomogeneities in the B 0  field. 
     
     
       6. The method of  claim 2  further comprising the step of determining a shimming factor for each of the number of target points based on the at least one of a field strength variation from the pre-defined B 0  field strength and an inhomogeneity variation from the minimum acceptable field inhomogeneity at each of the target points. 
     
     
       7. The method of  claim 6  wherein the step of determining a shimming factor includes the step of determining a lower bound vector and an upper bound vector, the lower bound vector and the upper bound vector representing minimum and maximum constraints of at least one of the pre-defined B 0  field strength and the minimum acceptable field inhomogeneity. 
     
     
       8. The method of  claim 7  further comprising the steps determining at least one of field contribution at each of the number of target points and/or spherical harmonics at each of the number of target points and determining a convergence vector for each target point such that multiplication of the convergence vector to the at least one of field contribution and spherical harmonics at each of the number of target points yields a product that falls between the lower bound vector and the upper bound vector. 
     
     
       9. The method of  claim 8  wherein the step of determining a convergence vector includes the step of determining a total amount of shimming required at each target point to achieve an actual B 0  field strength that at least approximates the pre-defined B 0  field strength and an actual field inhomogeneity that does not exceed the minimum acceptable field inhomogeneity. 
     
     
       10. A computer readable storage medium having a computer program stored thereon to develop a shimming model for a magnet assembly of an MR imaging system, the computer program representing a set of instructions that when executed by a computer causes the computer to: 
       map a B 0  field generated by an assembled magnet assembly designed to generate a B 0  field at a pre-shimming field strength;  
       identify a number of target points within the B 0  field; and  
       determine an amount of shimming required at each of the target points such that the pre-shimming field strength of the B 0  field is maintained simultaneously with substantial cancellation of inhomogencities within the B 0  field.  
     
     
       11. The computer readable storage medium of  claim 10  wherein the set of instructions further causes the computer to generate a matrix comprised of amounts of shim strength necessary at each of the target points such that the pre-shimming field strength of the B 0  field is maintained simultaneously with substantial cancellation of inhomogeneities within the B 0  field. 
     
     
       12. The computer readable storage medium of  claim 11  wherein the set of instructions further causes the computer to determine at least one of an active shim strength and a passive shim strength for each of the target points. 
     
     
       13. The computer readable storage medium of  claim 12  wherein the amounts of shim strength are either in terms of field contribution at each of the target points or spherical harmonics including B 0  contribution at each of the target points. 
     
     
       14. The computer readable storage medium of  claim 12  wherein the set of instructions further causes the computer to determine an amount of shimming material required at each of the target points such that the pre-shimming field strength of the B 0  field is maintained simultaneously with substantial cancellation of inhomogeneities within the B 0  field. 
     
     
       15. The computer readable storage medium of  claim 14  wherein the amount of shimming material depends upon shim type. 
     
     
       16. The computer readable storage medium of  claim 15  wherein the set of instructions further causes the computer to determine amount of magnetic material if the shim type is a permanent magnet, determine amount of iron to incorporate into the magnet assembly if the shim type is a passive shim, and determine amount of current to apply if the shim type is an active shim coil. 
     
     
       17. A method of manufacturing a magnet assembly for an MR imaging system, the method comprising the steps of: 
       constructing a permanent magnet assembly designed to generate a B 0  field at an average field strength and a desired field homogencity about a volume-of-interest (VOI);  
       determining variations in field homogeneity along the B 0  field; and  
       minimizing the variations in field homogeneity while maintaining the average field strength without mechanical adjustments to the permanent magnet assembly.  
     
     
       18. The method of  claim 17  wherein the step of minimizing includes the step of incorporating a number shimming elements into the permanent magnet assembly. 
     
     
       19. The method of  claim 18  further comprising the step of isolating a number of shimming points along the permanent magnet assembly and selectively adding at least one of an active shim element or a passive shim element at the number of shimming points. 
     
     
       20. The method of  claim 19  further comprising the step of determining an amount of shimming material required at each of the shimming points such that cumulative variations in the field strength are offset by cumulative amounts of shimming material. 
     
     
       21. The method of  claim 17  further comprising the step of determining an amount of field strength variation at each of the shimming points.